1. Field of the Invention
The present invention relates to gas delivery systems, such as pressure support systems and other ventilator (invasive or non-invasive) systems, and, more particularly, to a method for estimating the upper airway resistance of a subject using the gas delivery system, and a gas delivery system employing such a method.
2. Description of the Related Art
As is well known in the art, there are numerous situations where it is necessary or desirable to deliver a flow of breathing gas non-invasively to the airway of a patient, i.e., without intubating the patient or surgically inserting a tracheal tube in his or her esophagus. Such therapies are commonly referred to as non-invasive ventilation (NIV) therapies. For example, it is known to non-invasively deliver continuous positive airway pressure (CPAP) or variable airway pressure, which varies with the patient's respiratory cycle, to treat a medical disorder, such as sleep apnea syndrome, in particular, obstructive sleep apnea (OSA), or congestive heart failure. NIV therapies involve the placement of a patient interface device including a mask component on the face of a patient, wherein the patient interface device interfaces the ventilator or pressure support device with the airway of the patient. As is also well known in the art, there are also a number of situations where it is necessary or desirable deliver a flow of breathing gas to the airway of a patient invasively, i.e., wherein the patient is intubated or has a surgically inserted tracheal tube.
In providing ventilatory assistance to patients, such as in the various ventilation therapies described above, it is often helpful and/or necessary to be able to obtain an estimate of the upper airway resistance of the patient. However, estimating upper airway resistance in mechanically ventilated patients who have spontaneous respiratory efforts is rather complex, primarily due to that fact that knowledge of the force applied to the respiratory system is required and the fact that, in ventilated patients who have spontaneous respiratory efforts, that force includes a component related to pressure generated by respiratory muscles (Pmus), which continuously changes during the inflation phase of ventilation.
While there are a number of known methods for patient airway resistance measurement/estimation, including the well known interrupter and forced oscillation techniques, such methods have their drawbacks and limitations. In particular, such known methods can be adversely affected by non-ideal instrumentation and/or leaks in the patient circuit. There is thus room for improvement in the area of patient airway resistance measurement/estimation and a need for a system and method for effectively estimating airway resistance that is not adversely affected by non-ideal instrumentation and/or flow leaks.